Effects of CeO2 addition on the properties of cordierite-bonded porous SiC ceramics

Cordierite-bonded porous SiC ceramics were fabricated with and without CeO₂ addition in air by an in situ reaction bonding technique. The effects of CeO₂ addition on the phase composition, microstructure and properties of porous SiC ceramics were investigated. It was found that the CeO₂ addition strongly promotes the phase transformation towards cordierite while inhibits the formation of spinel. With CeO₂ addition, the neck growth was enhanced, and the mechanical properties of porous SiC ceramics were improved. When 2.0 wt.% CeO₂ was added, a flexural strength of 35.3 MPa was achieved at an open porosity of 42.1%. Pore size distribution characterization by mercury porosimetry indicated that 2.0 wt.% CeO₂ addition enlarged the average pore size of porous SiC ceramics and introduced a bigger homogeneity in pore size. In addition, it was found that higher permeability was obtained after adding 2.0 wt.% CeO₂. Moreover, the thermal shock resistance of cordierite-bonded porous SiC ceramics was also improved by the addition of CeO₂.     

Fabrication of SiC reticulated porous ceramics with multi-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with multi-layered struts were fabricated at 1450 °C by polymer sponge replica technique, followed by vacuum infiltration. The effect of additives (polycarboxylate, ammonium lignosulfonate and sodium carboxymethyl-cellulose) on the rheological behavior of silicon carbide slurry was firstly investigated, and then the slurry was coated on polyurethane open-cell sponge template. Furthermore, alumina slurry was adopted to fill up the hollow struts in vacuum infiltration process after the coated sponge was pre-treated at 850 °C. The results showed that the coating thickness on the struts and the microstructure in SiC RPCs were closely associated with the solid content of alumina slurry during vacuum infiltration. The typical multi-layered strut of SiC RPCs could be achieved after the infiltration of an alumina slurry containing 77 wt% solid content. The compressive strength and thermal shock resistance of the infiltrated specimens were significantly improved in comparison with those of non-infiltrated ones. The improvement was attributed to the in-situ formation of reaction-bonded multilayer struts in SiC RPCs, which were characterized by the exterior coating of aluminosilicate-corundum, middle part of mullite bonded SiC and interior zone of corundum.     

Fabrication and properties of SiC porous ceramics using a polyurethane preparation process

SiC porous ceramics can be prepared by introducing the polyurethane preparation method into the production process of ceramic biscuits, followed by sintering at 1300?°C for 2?h under N₂ flux after the cross-linking of polycarbosilane at 220?°C for 4?h in air. The microstructures, mechanical properties and infiltrations of the SiC porous ceramics are investigated in detail. The best dispersal effect comes from the SiC slurry with xylene as the solvent and a mixture of Silok®7096 (1?wt%) and Anjeka®6041 (4?wt%) as the dispersant. The compressive strength of SiC porous ceramics with high porosity (69.53%) reaches 16.9?MPa. The heat treatment can increase infiltration, the rate of which (4.296?×?10^?7 mm²) after the heat treatment at 750?°C in air is approximately two times faster than that before the heat treatment. The SiC porous ceramics fabricated in this study will have potential application in active thermal protection systems.     

Fabrication of porous SiC ceramics having pores shaped with Si grain templates

SiC porous ceramics were prepared by heating mixtures of Si powder and carbon black at 900 °C for 24 h in Na vapor. The grains of the Si powder were not only the source of Si for SiC but also served as templates for the pores in the SiC porous ceramics. Angular-shaped pores with sizes of 2-10, 10-150 and 50-150 μm were formed by angular Si grains with sizes of ≤10, ≤50 and ≤150 μm, respectively. The porosity of the SiC porous ceramics was around 55-59%. Spherical pores were also formed when spherical Si grains were used. A bending strength of 14 MPa was measured for the SiC porous ceramics prepared with the Si grains (≤50 μm).     

Fabrication and properties of C/SiC porous ceramics by grinding-mould pressing-sintering process

High temperature resistant porous ceramics are considered to be prime candidates for applications in the transpiration cooling system of a hypersonic vehicle. This paper describes a new preparation process including grinding-mould pressing-sintering process, which is successfully used to fabricate C/SiC porous ceramics with high compressive strength and excellent permeability. The effects of carbon fiber content on the microstructure, mechanical property, pore size distribution and permeability of this porous ceramic are investigated in detail. The results indicate that this porous ceramic prepared in this study exhibits high compressive strength (～270.82 MPa) and excellent permeability (～3.937 × 10^?8 mm²). The C/SiC porous ceramics fabricated in this study will have potential application in active thermal protection systems.     

Fabrication and characterization of porous CaSiO3 ceramics

Porous CaSiO₃ ceramics were prepared via a solid-state reaction method using CaCO₃ and SiO₂ as raw materials and active carbon as a pore-forming agent. The results indicated that porous CaSiO₃ ceramics could be obtained under a low sintering temperature of 1320?°C. The addition of active carbon significantly affected the volume density, microstructure, pore size distribution and mechanical strength of porous CaSiO₃ ceramics. With the increase of active carbon content, the volume density decreased, meanwhile the pore size and porosity increased gradually. Besides, the three-point bending tests demonstrated that the mechanical strength was decreased with increasing active carbon content. However, all the porous ceramics still exhibited high mechanical strength. These results implied that the increase of active carbon content not only enlarged the pore size and enhanced the porosity, but also kept a remarkable mechanical strength of porous CaSiO₃ ceramics. Therefore, these rationally designed CaSiO₃ porous ceramics will be a highly potential material in various applications due to its high mechanical strength, low sintering temperature and narrow pore size distribution.     

Low-temperature fabrication and characterization of porous SiC ceramics using silicone resin as binder

Commercially available silicone resin and silicon carbide (SiC) powders were adopted as the starting materials for the fabrication of porous SiC ceramics. During the heat treatment process, silicone resin experienced an organic–inorganic transformation and acted as the bonding material between SiC particles at a low temperature of 1000 °C. The mean particle size of starting SiC powders and silicone resin content can control the pore size, open porosity and fracture strength. The flexural strength of porous SiC ceramics increases with increasing silicone resin content and decreasing mean particle size of SiC powders. Larger pores can be obtained with coarser starting SiC powders and higher silicone resin content. The fracture surface of porous SiC ceramics was observed.     

生物SiC多孔陶瓷的研究进展

介绍了生物SiC多孔陶瓷的研究现状,并对生物碳模板的分类、制备方法进行了详尽的叙述,同时对生物SiC多孔陶瓷的各项性能进行了综述,最后对生物SiC多孔陶瓷的发展趋势作了总结。     

Formation of porous SiC ceramics via recrystallization

In this study, porous SiC ceramics with interconnected huge plate-like grains were fabricated from oxidized β-SiC powder with 1 wt% B₄C. When the β–α SiC phase transformation occurred at 2100 °C, rapid grain growth of α-SiC consumed the unstable β-SiC matrix resulting in an interconnected network structure with huge plate-like grains. The oxidation of β-SiC powder and the addition of B₄C are necessary conditions for rapid grain growth. The observed results are discussed based on thermodynamic considerations. The measured porosity of the specimens sintered at 2100 °C for 30 min was 47% and the mean pore size was 6–7 μm. The strength of the sintered specimen was 45 ± 5 MPa.     

Fabrication and characterization of porous cordierite ceramics prepared from fly ash and natural minerals

Low cost, single-phase porous cordierite ceramics are successfully synthesized by in-situ solid-state reactions from fly ash, quartz, and magnesite. The effects of sintering temperature and magnesite content on phase transformation, open porosity, bulk density, mechanical properties, and microstructure are carefully investigated. Factsage analyses are carried out to calculate the isopleth diagrams, and the results agree well with the experimental outcomes. Thermal gravimetric and differential scanning calorimeter analyses (TG-DSC) are performed to characterize the weight loss and transient behaviors of the raw materials. Linear thermal expansion properties are also studied. The α-cordierite phase is the only phase observed in S-3 (magnesite content 25%) sintered at 1300 °C for 2 h, and its linear thermal expansion coefficient (2.71 × 10⁻⁶ K⁻¹) is close to that of typical cordierite. Both the compressive strength (72.64 MPa) and flexural strength (23.92 MPa) of the as-synthesized samples are high with an open porosity of 33.16% and a bulk density of 1.61 g/cm³.     

Processing and properties of cordierite-silica bonded porous SiC ceramics

Cordierite-silica bonded porous SiC ceramics were fabricated by infiltrating a porous powder compact of SiC with cordierite sol followed by sintering at 1300–1400 °C in air. The porosity, average pore diameter and flexural strength of the ceramics varied 30–36 vol%, ~ 4–22 µm and ~ 13–38 MPa respectively with variation of sintering temperature and SiC particle sizes. In the final ceramics SiC particles were bonded by the oxidation-derived SiO₂ and sol-gel derived cordierite. The corrosion behaviour of sintered SiC ceramics was studied in acidic and alkaline medium. The porous SiC ceramics were observed to exhibit better corrosion resistance in acid solution.     

Fabrication and microstructure of porous SiC ceramics with Al2O3 and CeO2 as sintering additives

In the present study, porous silicon carbide ceramics were prepared via spark plasma sintering at relatively low temperatures using Al₂O₃ and CeO₂ as sintering additives. Sacrificial template was selected as the pore forming mechanism, and gelcasting was used to fix the slurry in a short time. The evolution process of the microstructures during different steps was observed by SEM. The influence of the sintering temperature and sintering additives on the shrinkage and porosity of the samples was studied. The microstructures of different samples were characterized, and the mechanical properties were also evaluated.     

碳化硅纳米纤维薄膜的制备及表征

通过溶胶-凝胶碳热还原法制备了超长碳化硅纳米纤维(SiCNF),并采用丙酮辅助滚压法制备了新型自支撑SiCNF薄膜。通过X射线衍射、场发射扫描电镜、红外光谱等测试手段,分析了SiCNF的形貌、晶相和成分,表明产物为直径约50nm的超长立方相β-SiCNF。自支撑纳米薄膜作为NF的宏观体材料,在柔性电子器件、复合材料、能源材料、高温过滤、高温催化载体、环境治理、生物工程等领域具有广阔的应用前景。     

Liquid phase sintering and characterization of SiC ceramics

The present study focuses on the sintering of silicon carbide-based ceramics (SiC) by liquid phase sintering (LPS) followed by characterization of the produced ceramics. AlN/Re₂O₃ mixtures were used as additives in the LPS process. In the first step, the LPS-SiC materials were produced in a graphite resistance furnace in the form of discs at different temperatures. The conditions with the best results regarding real density and relative density were taken as reference for sintering in the form of prismatic bars. In the second step, these samples were evaluated regarding fracture toughness (K_IC), by the Single Edge V Notch Beam – SEVNB – method, and flexural strength. K_IC behavior was evaluated according to the depth and curvature radius of the notches. Reliable K_IC values were presented when the ceramic displayed a small curvature radius at the notch tip. When the radius was large, it did not maintain the square root singularity of the notch tip. Tests were carried out to determine K_IC values in atmospheric air and water. K_IC results were lower in water than air, with a decrease ranging between 2.56% and 11.26%. The observations indicated a direct grain size correlation between K_IC values and fracture strength of the SiC ceramics.     

Spheroidization of SiC powders and their improvement on the properties of SiC porous ceramics

Spherical SiC powders were prepared at high temperature using commercial SiC powders (4.52 µm) with irregular morphology. The influence of spherical SiC powders on the properties of SiC porous ceramics was investigated. In comparison with the as-received powders, the spheroidized SiC powders exhibited a relatively narrow particle size distribution and better flowability. The spheroidization mechanism of irregular SiC powder is surface diffusion. SiC porous ceramics prepared from spheroidized SiC powders showed more uniform pore size distribution and higher bending strength than that from as-received SiC powders. The improvement in the performance of SiC porous ceramics from spheroidized powder was attributed to tighter stacking of spherical SiC particles. After sintering at 1800 °C, the open porosity, average pore diameter, and bending strength of SiC porous ceramics prepared from spheroidized SiC powder were 39%, 2803.4 nm, and 66.89 MPa, respectively. Hence, SiC porous ceramics prepared from spheroidized SiC powder could be used as membrane for micro-filtration or as support of membrane for ultra/nano-filtration.     

SiC多孔陶瓷制备方法研究进展

碳化硅(SiC)多孔陶瓷作为一种重要的结构材料,具有高熔点、高强度、比表面积大、体积密度小、热膨胀系数小以及良好的化学稳定性等优点,被广泛应用于催化剂载体、气/液过滤装置、生物医学材料、保温材料和耐火材料等领域。SiC多孔陶瓷的微观结构、性能及服役寿命等均受其制备方法的影响,因此综述了近年来国内外在SiC多孔陶瓷制备方法方面的研究进展,总结了物理成孔法(包括颗粒堆积法、冷冻干燥法及3D打印法等)和化学成孔法(包括添加造孔剂法、有机泡沫浸渍法与生物模板法等)制备SiC多孔陶瓷的优缺点,并对其发展方向和重点进行了展望。     

Fabrication and characterization of SiC whisker reinforced reaction bonded SiC composite

SiC whisker reinforced reaction bonded SiC composites have been developed for fabricating large scale, complex shaped structural components. Here the composites were prepared with the method of slip casting and liquid silicon infiltration at 1650 °C. The distribution, morphology and reinforcing behaviors of the SiC whisker in the composite were investigated. It is revealed that the introduction of SiC whisker increases the porosity of the green body, and accordingly the bulk density of the composite. Whisker pullout can be clearly observed on the fracture surface, implying a moderate bonding strength between the whisker and matrix. After liquid silicon infiltration, the SiC whisker keeps its initial diameter and morphology in the case of 15 wt% whisker. The fracture toughness is enhanced by SiC whisker, reaching the peak value of 4.2 MPa m^1/2 at the whisker fraction of 20 wt%. Whisker pullout, whisker bridging and crack deflection are considered as the main toughening mechanisms.     

Fabrication of wood-like porous silicon carbide ceramics without templates

The porous silicon carbide ceramics with wood-like structure have been fabricated via high temperature recrystallization process by mimicking the formation mechanism of the cellular structure of woods. Silicon carbide decomposes to produce the gas mixture of Si, Si₂C and SiC₂ at high temperature, and silicon gas plays a role of a transport medium for carbon and silicon carbide. The directional flow of gas mixture in the porous green body induces the surface ablation, rearrangement and recrystallization of silicon carbide grains, which leads to the formation of the aligned columnar fibrous silicon carbide crystals and tubular pores in the axial direction. The orientation degree of silicon carbide crystals and pores in the axial direction strongly depends on the temperature and furnace pressure such as it increases with increasing temperature while it decreases with increasing furnace pressure.